1. Field of the Invention
The present invention relates to a shutter time control circuit for compensating the exposure error due to the overlapping amount of the focal plane shutter curtains and so on.
2. Description of the Prior Art
Generally, in case of a camera with focal plane shutter the leading shutter curtain is let to start with the shutter release while the timer circuit for controlling the exposure time is actuated so as to carry out the clock operation, while the tail shutter curtain is let to start at the time at which the exposure time is clocked by means of the timer circuit in order to control the exposure time. However, in case of the focal plane shutter it is necessary to overlap the leading shutter curtain on a part of the tail shutter curtain at the time of the shutter charging as well as before the start of the operation, while these unavoidably exists are response delay of the tail shutter curtain holding magnet, so that it is possible that the clocked exposure time could correspond to the actual film exposure time due to the difference between the starting position of the leading shutter curtain and that of the tail shutter curtain or the response delay of the magnet. Consequently, the clock operation is started by means of the timer circuit along with the start of the leading shutter curtain while the tail shutter curtain is let to start at the time at which the clock operation by means of the timer circuit has been completed, in accordance to which a precise exposure control is impossible. Namely, in case of the focal plane shutter, as is shown in FIGS. 1(a) and (b), the starting position FS1 at which the end of the leading shutter curtain starts and the starting position rs1 at which the front of the tail shutter curtain starts are deviated from the exposure opening position a1 toward a2. Consequently, even if the leading shutter curtain is let to start to run at the time point t1, the clock operation is started at the time point t1 as is shown in FIG. 1(c) and the tail shutter curtain holding magnet is actuated at the time point t2 after the lapse of the exposure time Te so as to allow the tail shutter curtain to start to run, the tail shutter curtain only starts to run at the time point t3 after the lapse of the response delay Tmg of the tail shutter holding magnet, while further there exists a time difference Tg due to the difference of the starting points between the leading and the tail shutter curtain, so that the actual film exposure time TA becomes shorter than the clocked time Te, namely does not correspond to the clocked time Te in such a manner that the exposure time cannot be controlled in a precise way. Namely, as is clear from FIGS. 1(b) and (c), there exists a relation TA+Tg=Te+Tmg among the response delay Tmg of the magnet, the overlapping time Tg of the shutter curtains, the clocked time Te and the actual film exposure time TA, namely TA=Te+Tmg-Tg and TA.noteq.Te. Consequently, even if the time is clocked precisely, it is impossible to control the actual film exposure time precisely. Namely, as is clear from FIGS. 1(b) and (c), the actual film exposure time TA is shorter than Te by Tg-Tmg, so that unless the clocked time should be prolonged by Tg-Tmg as is shown in FIG. 1(d), it would be impossible to control the exposure time precisely.
In order to overcome such difficulties, several methods have been proposed, as is disclosed for example out of U.S. Pat. Nos. 3,683,767 and 3,721,166, in accordance to which beside the clock circuit a CR timer circuit is provided or a resistance is connected in series with the timer circuit consisting of a photoelectric element and a condenser so as to prolong the clock time. However, the above method is suited for the clock circuit constituting the analog control electronic shutter consisting of a photoelectric element such as CdS and so on and a condenser, but not to the so called digital control electronic shutter for controlling the shutter so designed that the clock pulses are counted by means of a counter so as to deliver a signal for closing the shutter when the value counted by means of the counter has reached a shutter time value corresponding to the brightness or the like. Further, DOLS 2440679 has proposed a camera to designed that the digital control electronic shutter is provided with a delay circuit in such a manner that after the lapse of the time determined by means of the delay circuit the counter starts counting operation so as to prolong the actual clocked time in order to eliminate the above difficulties. However, even in case of the above method it is necessary to make use of a one shot circuit as delay circuit, so that there still exists a problem for the matching with the digital control camera.